Correction: An earlier version of this article incorrectly said that graphite stoppers in the reactor cores would begin to melt at 350 degrees Fahrenheit. Graphite has a high melting point, about 6,600 degrees Fahrenheit. The stoppers would not melt at 350 degrees but could begin to “degrade,” according to nuclear engineer Arnie Gundersen. This version has been corrected.

TOKYO — As radiation levels at the Fukushima Daiichi nuclear plant reached a new high Sunday, workers contended with dark, steamy conditions in their efforts to repair the facility’s cooling system and stave off a full-blown nuclear meltdown. Wearing respirators, face masks and bulky suits, they fought to reconnect cables and restore power to motor pumps the size of automobiles.

Leaked water sampled from one unit Sunday had 100,000 times the radioactivity of normal background levels, although the Tokyo Electric Power Co., which operates the plant, first calculated an even higher, erroneous, figure it didn’t correct for hours.

Tepco apologized Sunday night when it realized the mistake; it had initially reported radiation levels in the leaked water from the unit 2 reactor as being 10 million times the norm, which prompted an evacuation of the building.

After the levels were correctly measured, airborne radioactivity in the unit 2 turbine building still remained so high — 1,000 milli­sieverts per hour — that a worker there would reach his yearly occupational exposure limit in 15 minutes. A dose of 4,000 to 5,000 millisieverts absorbed fairly rapidly will eventually kill about half of those exposed.

The latest confusion in the operation to stave off a full-scale nuclear meltdown at the crippled facility underscores the immense challenges for the several hundred workers in a desperate battle to restart the critical cooling systems. Seventeen workers have been exposed to high levels of radiation, including three who were hospitalized last week, as technicians conducted highly nuanced electrical work in dark conditions that one nuclear industry expert termed “hellish.”

Japanese authorities say efforts to control Fukushima’s overheated reactors will take months and during that time radiation will continue to leak into the environment, extending a nuclear emergency that already ranks as the world’s most serious in a quarter-century. Several hundred workers now shoulder the responsibility for limiting the crisis, amid potentially lethal radiation levels, and on Saturday the chief of Japan’s nuclear agency called on Tepco to improve its worker safety.

Police stationed in the area have noticed more people returning to gather belongings and “there is a risk” of returning home now, Edamo said. Many families fled quickly after the earthquake and tsunami struck more than two weeks ago with only the clothes they were wearing.

Evidence of rising contamination in and around the plant has tempered optimism from a week ago, when engineers began work to restore power to the first of the damaged reactor buildings. Japan’s Nuclear and Industrial Safety Agency said Sunday that a new measurement of seawater taken about 1,000 feet from the facility showed an iodine level 1,850.5 times the legal limit, higher than a reading taken the previous day.

The dangers in unit 2 merely add to the growing challenges. Radioactive water is pooling in four of Fukushima’s six turbine rooms, and engineers have no quick way to clean it up, although they have begun to try in unit 1.

While a Tepco spokesman said Sunday that he did not know how the radioactive water was leaking from the reactor cores, Yukio Edano, chief cabinet secretary, said in a televised interview Sunday morning that the reactor itself had not been breached.

He said it was clear that water that could have been inside the unit 3 reactor had leaked but the reactor had not been breached. Still, he said, “Unfortunately, it seems there is no question that water, which could have been inside the reactor, is leaking.’’

Unlike in newer reactor designs, the older boiling-water reactors at Daiichi are pierced by dozens of holes in the bottoms of their reactor vessels. Each hole allows one control rod — made of a neutron-absorbing material that quickly stops nuclear fission inside the reactor — to slide into the reactor from below, as happened when the earthquake shook the plant March 11. During normal operations, a graphite stopper covers each hole, sealing in highly radioactive primary cooling water, said Arnie Gundersen, a consultant at Fairewinds Associates with 40 years of experience overseeing boiling-water reactors.

But at temperatures above 350 degrees Fahrenheit, the graphite stoppers begin to degrade.

“Since it is likely that rubble from the broken fuel rods . . . is collecting at the bottom of the reactor, the seals are being damaged by high temperature or high radiation,” Gundersen said. As the graphite seals fail, water in the reactor will leak into a network of pipes in the containment buildings surrounding each reactor — the very buildings that have been heavily damaged by explosions. Gundersen said that this piping is probably compromised, leaving highly radioactive water to seep from the reactor vessels into broken pipes — and from there into the turbine buildings and beyond.

To stabilize the facility, workers are trying to repair the elaborate cooling system, necessary to keep the reactor cores and spent fuel pools from overheating. For now, they are conducting this work in dark, steamy conditions. Nuclear safety experts say they must shift out of the most dangerous areas every 30 minutes to an hour, to prevent radiation overexposure.

Meanwhile, they are racing to repair motor pumps. Their environment resembles a cavern of cables. Some of the equipment was damaged during the March 11 earthquake and tsunami. Other equipment has been corroded by salt water, which was poured into the facility during earlier efforts to cool the reactors.

“To a layman, you’d be scared to death,” said Lake Barrett, a nuclear engineer who directed the cleanup of Three Mile Island. “You’re working with salt water around your feet. This is not the way electricians usually work.”

The number of workers at the Daiichi plant fluctuates from day to day, ranging between 500 and 1,000. But Tepco employees account for only a part of the labor force. Last Tuesday, for instance, there were 700 people at the plant, a nuclear agency official said. The figure included 500 Tepco employees, 100 subcontracted workers, and 100 members of Japan’s Self-Defense Forces or the Tokyo Fire Department.

One subcontracted worker who laid cables for new electrical lines March 19 described chaotic conditions and lax supervision that made him nervous. Masataka Hishida said neither he nor any of the workers around him was given a dosimeter, a device used to measure one’s exposure to radiation. He was surprised that workers were not given special shoes; rather, they were told to put plastic bags over their street shoes. When he was trying on the gas mask for the first time, he said, the supervisor told him and other subcontractors, “Listen carefully, I’m only going to say this one time,” while explaining how to use it.

When Hishida finished his work shift, an official scanned his whole body for radiation. He came up clean, except for the very tip of his beard. He was sent into a shower where he lathered up and scrubbed his beard. He was tested again and passed.

A few days later, still worried about the extent of his radiation exposure, he trimmed his beard.

Staff writers Michael Alison Chandler and David Nakamura and special correspondents Kyoko Tanaka, Akiko Yamamoto and Kato Tetsuya contributed to this report.

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